1. Field of the Invention
The present invention relates to transparent substrates, particularly of glass coated with a stack of thin layers having at least one metallic layer enabling it to act upon solar radiation and infra-red radiation of long wavelength.
2. Discussion of the Background
One known type of a stack of thin layers for providing transparent substrates with thermal properties, particularly with low emissivity, is formed mainly by a metallic layer, particularly of silver, disposed between two coatings of dielectric material based upon a metal oxide. This type of stack is generally made by a succession of depositions carried out using a vacuum with cathodic sputtering assisted by a magnetic field. Two very thin metallic layers can also be provided on both sides of a silver layer, the underlying layer acting as a protective or "sacrificial" layer so as to avoid oxidizing the silver, if the oxide layer overlaying it is deposited by reactive sputtering in the presence of oxygen.
If the silver layer essentially determines the thermal anti-solar and/or low emissivity performance of the final glazing, the layers of dielectric material fulfill several roles, because they act in the first place on the optical aspect of the glass in an interferential manner. They also protect the silver layer from chemical and/or mechanical damage. Thus, French Patent FR-B-2 641 271 describes a stack where the silver layer is intercalated between two coatings of dielectric material, such of the coatings being formed of a plurality of layers of metallic oxide. The underlying coating of the silver layer is formed of the oxide layers superimposed on a layer of stannic oxide, that layer adjacent to the silver layer being of zinc oxide and having, according to this patent, the effect of protecting the silver, notably by making it less vulnerable to attack by oxygen. On the other hand, the thickness of the zinc oxide layer is small, because the zinc oxide, which is hardly resistant would endanger, if it were too great, the whole assembly of the stack. The layers of dielectric material which surround the silver layer protect it in this way against attack and can also optimize its quality for improving its priming or wetting, as described in European Patent Application No. EP-A-0 611 213.
Currently more and more low emissivity or anti-solar functional glazing is required having, in addition, substrates with inherent characteristics, particularly anesthetic (which may be curved), having mechanical characteristics (which may be more resistant) or for safety glass (which does not cause injury when broken).
This requires that the glass substrates be subjected to thermal treatment for bending, annealing or tempering. If this is carried out with conventional stack coating without any precaution or adaption of the thin layers, the silver layer tends to be irreversibly degraded, completely deteriorating the thermal properties thereof. This is for several reasons, however, primarily because of the heating effect, the silver layer is oxidized by diffusion of the atmospheric oxygen across the layers which overlay it. It also tends to be oxidized by diffusion of the oxygen of the glass across the underlying layers. Lastly, it may tend to be excessively altered in contact with alkaline ions, such as sodium (Na.sup.+) migrating from the glass across the underlying layers. The diffusion of the oxygen or alkaline ions can be facilitated and amplified by the deterioration or the structural modification of the oxide layers themselves caused by the heating effect.
A first solution is to increase very significantly the thicknesses of the thin layers previously mentioned, on either side of the silver layer. If sufficiently tick they can effectively "screen" and protect the silver layer. If it also manages to practically preserve the thermal properties of the stack unchanged, particularly as regards its emissivity, on the other hand by modifying the optical properties, the two metal layers are oxidized mainly because of the silver layer, and they cause, particularly, a great increase in the light transmission T.sub.L.
Low emissivity tempered glazing can also be obtained after the deposition of the layers having a T.sub.L value greater than 80%, which is clearly inferior to this value before tempering. It can also be particularly noted in European Patent Application No. EP-A-0 506507 that a temperable stack is described, with one silver layer disposed between a layer of tin and a layer of nickel-chrome-chrome. However, it is clear that having been tempered, the coated glass with such a stack was until then considered to be a semi-finished product of no use as such, since its light transmission value of about 60% to 70% was hardly compatible with the current market for low emissivity highly transparent glazing.
The inconvenience which inevitably follows is that it is, therefore, absolutely necessary to provide at the same time, two types of stack having low emissivity layers and/or anti-solar layers, one for non-tempered glass and the other for glass destined to be tempered or bent, and which is also complicated regarding research and development requirements, particularly, for the management of production stock.